When silicon wafer edges are abrasively polished using present methods, silicon and abrasive particles adhere to the surfaces of the wafer and are difficult to remove.
Also, when silicon wafer edges are chem-mechanically polished, as in U.S. Pat. No. 5,128,281, the wafer surfaces adjacent to the edges etch slightly because of attack by the warm alkaline polish slurry.
Prior art methods have applied films such as photoresist or wax to protect on the back of the wafer. These methods have disadvantages in that materials, equipment and labor are expensive to apply and remove the films. In later processing, the wafer cannot be polished sufficiently flat with the layer in place, and the methods do not protect against "slurry rings ". The abrasive edge polish process sometimes lifts a portion of the film away from the wafer, allowing etching under the film in later processing. Wax or photoresist protection involves the extra steps of application and removal.
Another process used in prior art methods of particle removal is to etch the wafer with an acid or caustic solution after abrasive edge polishing. This has the disadvantage of removing extrinsic gettering sites (backside damage).
Prior art mounting systems predominantly used for single sided polishing of semiconductor wafers are template mounting and backside wax mounting. Template mounting involves the mounting of a template assembly, composed of a poromeric backing material which contacts the backside of the wafer, and a fiberglass/urethane retainer which surrounds the outer diameter of the wafer. The assembly is mounted on a polishing block. The poromeric backing film compresses non-uniformly during the polishing process, and high quality flatness and taper control can not be achieved. The film also increases greatly the cost of templates and reduces productivity.
In backside wax mounting, a layer of wax approximately 4 mils in thickness is deposited on the backside of the wafer, then the wafer is mounted onto a polishing block. Following the polish process, the wafer is demounted from the block and the wax is stripped from the wafer. Four problems with the wax mounting process are: (1) the uniformity of the wax layer is critical to achieving high quality flatness; (2) the wax residues contribute to high particles on the wafer; and (3) particles which are entrapped in the wax layer cause high points, or "dimples " in the polished wafer surface; and there is a yield loss associated with the demounting and the other steps.